An active matrix substrate used in liquid crystal display devices and the like has a switching element such as a thin-film transistor (hereinafter, “TFT”) in each pixel. Conventionally, a TFT with an amorphous silicon film as an active layer (hereinafter, “amorphous silicon TFT”) or a TFT with a polycrystalline silicon film as an active layer (hereinafter, “polycrystalline silicon TFT”) has been widely used as such a switching element.
There have been recent attempts to use materials other than amorphous silicon and polycrystalline silicon for the active layer of the TFT. A liquid crystal display device is disclosed in Patent Document 1 that uses an oxide semiconductor film such as InGaZnO (an oxide made of indium, gallium, and zinc; hereinafter, “IGZO”) to form the active layer of the TFT, for example. Such a TFT is called an “oxide semiconductor TFT.”
It is possible to operate the oxide semiconductor TFT at higher speeds than the amorphous silicon TFT. Furthermore, the oxide semiconductor film is formed with a process that is simpler than for the polycrystalline silicon film, and thus, the oxide semiconductor film can be applied to devices requiring a large area. Therefore, it is expected that the oxide semiconductor TFT will be used in emerging liquid crystal display devices that perform quadruple speed driving and 3D display as an active element that can perform high efficiency switching operation and that can be manufactured while suppressing the number of manufacturing steps and cost.
The electron mobility of the oxide semiconductor is also high, and thus, even if the size thereof is reduced, it is possible to have the same or better performance compared to a conventional amorphous silicon TFT. Thus, if using the oxide semiconductor TFT, the amount of area the TFT occupies in the pixel area of the liquid crystal display device or the like can be reduced, and as a result, the pixel aperture ratio can be improved. Accordingly, it is possible to have a brighter display and to achieve a reduction in power consumption due to the amount of light from the backlight being reduced. In particular, it is not easy to increase the pixel aperture ratio through minimum-width limitations for the wires (process rules) or the like in small-sized/high resolution display devices used in smartphones and the like. This is why the adoption of oxide semiconductor TFTs is expected.
When using an oxide semiconductor film, however, there is a risk that oxygen defects will create electron carriers in the TFT manufacturing process, such as in a heat treatment step, for example, causing a reduced resistance. In TFTs with a top-contact structure, the oxide semiconductor film at the bottom thereof is susceptible to damage during the etching step of the source and drain electrode and the formation step of the interlayer insulating film.
Thus, there are cases in which it has been difficult to achieve stable TFT characteristics when using an oxide semiconductor film as an active layer of the TFT. A top-contact structure refers to a structure in which the top of the semiconductor film, which functions as the channel, is connected to the source electrode and drain electrode. Normally, this structure is obtained by forming an island-like semiconductor layer and then forming a source/drain metal layer on the semiconductor layer.
A technology is disclosed in Patent Document 1 in which the OFF current of the TFT is reduced by conducting an oxidation treatment (an oxygen radical treatment using oxygen plasma) on a channel region of an oxide semiconductor TFT having this top-contact structure. This treatment increases oxygen concentration on the surface (sometimes called the back channel section) of the channel region that is opposite to the gate electrode, and forms an oxygen excessive region. This results in being able to increase the resistance of the back channel section and being able to reduce the OFF current in n-type oxide semiconductor TFTs made of IGZO.